118 ORGANIC EVOLUTION CONSIDERED 



make their appearance in the Silurian system. They 

 consist of the remains of fishes, the most determinable 

 of which are the plates of placoderms (Pteraspis, 

 Cephalaspis, Auchenaspis, Scaphaspis). The bone- 

 bed of the Ludlow rocks has also yielded certain 

 curved spines which have been referred to a Cestra- 

 cious, and some shagreen-like plates which have been 

 supposed to be scales of placoid fishes (Sphagodus, 

 Thelodus), and bodies like jaws with teeth which 

 were called Plectrodus, but which are now known to 

 be lateral shield-spines of a cephalaspidean fish 

 (Eukeras.pis)." * 



Recent discoveries have, therefore, carried the geo- 

 logical history of fishes from the upper part of the 

 Upper Silurian into the Lower Silurian. The evi- 

 dence seems to justify the conclusion that in the lower 

 part of the Trenton each of the three orders of Paleo- 

 zoic Fishes was well represented. This oldest known 

 evidence shows that Fishes had become so widely 

 different in structure that they can be classed in the 

 three great orders which include most of the fishes of 

 all geological time. 



This being true, if they were evolved, it is evident 

 that they must have had a long history which extends 

 back through the Primordial into the Archaean. It 

 will be remembered that the oldest known fossils of 

 any kind are in the Primordial. Already the known 

 history of Fishes extends back almost to that period. 

 Instead of their becoming more alike as we trace them 

 back in time, their earliest known remains prove them 

 to have been widely different in structure. This fact 

 renders more difficult the belief that they could have 

 been evolved, while it renders quite certain the fact 

 that if they were evolved they must have begun as 

 fishes at some remote period in the Archsen. 



* Text-Book of Geology, Third Edition, p. 744. 



